When endoprostheses are implanted, especially shell-shaped prostheses or parts of prostheses, respectively, it is necessary, especially in hip joints, for creating suitable fitting areas, to prepare the existing natural shape of the bone section by a chip-removing process. This is done so that the bone or bone section, respectively, which is provided with the prosthesis or the part of the prosthesis, is geometrically adapted to the shape of the prosthesis or the part of the prosthesis.
To attach a shell prosthesis on the femur head of a hip joint, first it is necessary to provide a cut, which is usually circular, on the front side of the femur head, which cut is adapted to the inner side of the prosthesis.
Afterwards, the femur head has to be worked at its outer side, which is adjacent to the front side, in order to create a suitable fitting area for the jacket of the shell prosthesis.
For shell prostheses which are known per se, it is sufficient when the above-mentioned outer side of the bone is cylindrically cut. However, to improve these known shell prostheses having cylindrical jackets, there have been developed shell prostheses whose jacket surfaces are not cylindrical, but conically tapered towards the free edge. Accordingly, for mounting such a shell prosthesis, it is necessary to provide a conical area which tapers from the outside on the bone or bone section to be treated. In the case of a human hip joint, this is the femur head which can be pre-worked cylindrically.
Such a treatment obviously results in considerable problems because the fitting areas, which had been created in advance, may not be damaged during this treatment. Moreover, the basic principle has always to be adhered to that only as little tissue as possible should be removed in order to reduce the extent or seriousness of the operation and not to lose the advantages which are achieved with shell-shaped prostheses as compared to total-endoprostheses.
A primary object of the present invention is to create an apparatus of the type described with which a conical area which tapers from the outside, is produced on a bone or bone section, especially the femur head of a human hip joint, without damaging the rounded fitting area (usually prepared in advance) on the front side of the bone. The conical area which is to be produced must reliably be axis symmetrical in order to guarantee an optimum adaptation to the geometric form of the existing shell prosthesis and, thus, to use only as little amount of bone cement as possible. Removal of portions of tissue, whose removal is not necessary, must be avoided. Further, it should be possible to treat the bone as quickly as possible, in order to keep the time of the operation as short as possible.